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SUMMARY:High 3He/4He in proto-Iceland plume basalts: implications for the 
 deep Earth - Fin Stuart\, SUERC\, East Kilbride
DTSTART:20100126T163000Z
DTEND:20100126T173000Z
UID:TALK20953@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:Intra-plate basalt volcanism derived from the largest mantle p
 lumes appear to be characterized by 3He/4He that are typical higher than m
 id-ocean ridge basalts (MORB).  There is little argument that this reflect
 s the contribution from a deep mantle reservoir that has experienced less 
 degassing of the primordial noble gases (and therefore all volatiles) than
  the convecting upper mantle. A convectively-isolated lower mantle appears
  to satisfy several global geochemical observations and was the favoured e
 xplanation for many years.  However this difficult to reconcile with geoph
 ysical observations and alternatives models have been proposed\, including
  isolated blobs in a whole convecting mantle\, a discrete core-mantle boun
 dary layer and\, most recently\, a convectively slow deep mantle.  In thes
 e models high 3He/4He is associated with a geochemically-distinct mantle r
 eservoir that is less depleted than the convecting upper mantle. \nWe have
  previously identified the highest terrestrial 3He/4He (up to 50 Ra) in th
 e earliest basalts erupted by Iceland plume.  Contrary to expectation thes
 e picrites exhibit lithophile radiogenic isotope and trace element composi
 tions that span nearly the complete range of global MORB and OIB.  This is
  in strong contrast to the expectation that the high 3He/4He basalts are d
 erived from a primordial gas-rich\, compositionally unique mantle reservoi
 r.  Noble gas (Ne\, Ar and Xe) isotope compositions cover the range of val
 ues that are typical of MORB and OIB\, consistent with the radiogenic isot
 ope and trace element data but strongly decoupled from He isotopes.  3He/2
 1Ne and 3He/36Ar are 1-2 orders of magnitude higher than measured in MORB 
 and OIB indicating that the initial stages of the plume sampled mantle tha
 t was enriched in high 3He/4He helium.  In this presentation I will review
  the possible mechanisms and implications of these observations.\n
LOCATION:Harker 1 seminar room\, Department of Earth Sciences
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